System for detecting wet and icy surface conditions

ABSTRACT

A system for detecting wet and icy conditions on the surface of highways, airport runways and the like. A first capacitor is positioned on a surface the condition of which is being detected. This capacitor has first and second spaced-apart electrodes which are positioned substantially coplanar with the surface and exposed to atmospheric precipitation which affects the capacitor&#39;&#39;s dielectric and capacitance. A second capacitor having first and second spaced-apart electrodes is positioned so as not to be exposed to atmospheric precipitation. Respective out-ofphase time-varying signals are applied to the first electrodes of said capacitors and the second electrodes are commonly connected. The system further includes a conductivity sensor having first and second spaced-apart electrodes exposed to atmospheric precipitation which affects the sensor&#39;&#39;s resistance, a sensor circuit which supplies an output voltage the magnitude of which is a function of the resistance of the sensor, and a logic circuit responsive to any signal coupled to the second electrodes of the capacitors reaching a predetermined precipitation threshold magnitude and to the output voltage of the sensor circuit reaching a predetermined ice threshold magnitude to provide an output which indicates an icy surface condition.

United States Patent 1 1 Overall SYSTEM FOR DETECTING WET AND ICYSURFACE CONDITIONS Primary Examiner-Andrew J. James AssistantExaminer-B. P. Davis Attorney, Agent, or Firm-Koenig, Senniger, Powers 1Mar. 25, 1975 [57] ABSTRACT A system for detecting wet and icyconditions on the surface of highways, airport runways and the like. Afirst capacitor is positioned on a surface the condition of which isbeing detected. This capacitor has first and second spaced-apartelectrodes which are positioned substantially coplanar with the surfaceand exposed to atmospheric precipitation which affects the capacitorsdielectric and capacitance. A second capacitor having first and secondspaced-apart electrodes is positioned so as not to be exposed toatmospheric precipitation. Respective out-of-phase time-varying signalsare applied to the first electrodes of said capacitors and the secondelectrodes are commonly connected. The system further includes aconductivity sensor having first and second spaced-apart electrodesexposed to atmospheric precipitation which affects the sensorsresistance, a sensor circuit which supplies an output voltage themagnitude of which is a function of the resistance of the sensor, and alogic circuit responsive to any signal coupled to the second electrodesof the capacitors reaching a predetermined precipitation thresholdmagnitude and to the output voltage of the sensor circuit reaching apredetermined ice threshold magnitude to provide an output whichindicates an icy surface condition.

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SYSTEM FOR DETECTING WET AND ICY SURFACE CONDITIONS BACKGROUND OF THEINVENTION This invention relates to detection systems and moreparticularly to systems for detecting wet and icy conditions on thesurface of highways, airport runways and the like. I

The desirability of having a reliable system for determining andindicating hazardous surface conditions 1 due to atmoshericprecipitation is self-evident. Such a system is useful to warn driversof wet, icy and incipiently icy conditions on highways and bridges, toalert maintenance crews of the need for application of deicingchemicals, and in the detection of icing on aircraft surfaces toindicate to the pilot that precautionary measures should be taken.

A number of systems have been proposed for detecting wet and icyconditions on a surface but each has one or more serious shortcomings.Such systems can be categorized as indirect or direct. Indirect systemsdo not monitor the actual surface condition but instead attempt topredict the surface conditions by measuring air temperature andhumidity. Needless to say, they are not reliable because ambienttemperature and humidity are not consistently accurate indicators of thepresence or absence of ice on an exposed surface. Some of the directmeasurement systems, such as those requiring microwave or betaradiation, are quite expensive and are not wholly reliable.

Most of the simple and less expensive direct systems usually depend onthe different conductivities of ice and water by using two pairs ofexposed adjacent electrodes with one pair being heated. This type ofdirect system often includes a temperature sensor and in one instanceincludes a sensor-capacitor to ascertain the amount of precipitationconnected as one leg of a bridge circuit as shown in US. Pat. No.3,428,890. Another direct system uses a heated stainless steel stripwith a thermocouple which is cyclically heated and cooled and senses thepresence of ice due to heat needed in the change of state from ice towater.

A serious drawback of these prior art direct systems is that a heatedsensor may melt the ice in its immediate vicinity and the resultingwater will evaporate or be blown or splashed off the heated sensor bypassing vehicles. With the water removed from the heated sensor bothsensors measure low conductivity thereby erroneously indicating thesurface to be clear.

A weather detector system using a series of exposed parallel verticalplates as a capacitance sensor was developed by the Navy to determineboth the rate of precipitation as well as the type. The dielectricconstant in the gaps between the plates would be affected by rainfalling downwardly therebetween the plates or by ice being formed orsnowfall, but this was unsatisfactory and impractical. Capacitancesensors have also been used in the insulation industry for measuring themoisture content of a sample of insulation such as paper. One suchsystem, shown in US. Pat. No. 3,684,953, uses a probe having upstanding,flat facing electrodes with the sample to be tested placed to bridge thetop edges ofthe electrodes but this, too, would be impractical fordetecting wet and icy conditions on the surface of a highway or runway,etc.

SUMMARY OF THE INVENTION Among the several objects of this invention maybe noted the provision of an improved direct system for detection of wetand icy surface conditions; the provision of such a system which willoperate reliably under all surface conditions and will not affect thestate or nature of the precipitation being detected by melting and- /orevaporation; the provision of a surface condition 0 detection systemwhich indicates clear, wet, or icy surface conditions and also theincipient formation of ice on the surface; the provision of such asystem which will accurately and reliably indicate surface conditionseven when ice control chemicals are present; the provision of such asystem in which the sensors are rugged and compact and easily mountableon the surface of a highway, runway or the like; and the provision ofsuch a system which employs high reliability solid-state electronics andis simple and economical to manufacture. Other objects and features willbe in part apparent and in part pointed out hereinafter.

Briefly a system for detecting wet and icy surface conditions includes afirst capacitor positioned on a surface the condition of which is to bedetected. The capacitor has first and second spaced-apart electrodeswhich are positioned substantially coplanar with the surface and exposedto atmospheric precipitation which affects the capacitors dielectric andcapaci tance. A second capacitor having first and second spaced-apartelectrodes is positioned so as not to be exposed to atmosphericprecipitation. Respective out-ofphase time-varying signals are appliedto the first electrodes of the capacitors, and the second electrodes arecommonly connected. A sensor circuit comprising a conductivity sensorhaving first and second spaced apart electrodes is positioned to beexposed to atmospheric precipitation thereby to affect the resistancethereof and supply and output voltage the magnitude of which is afunction of the resistance of the conductivity sensor. The systemfurther includes a logic circuit responsive to any signal coupled to thecommonly connected second electrodes of the capacitors reaching apredetermined precipitation threshold magnitude and to the outputvoltage of the sensor circuit reaching a predetermined ice thresholdmagnitude to provide an output which indicates an icy surface condition.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective of a sensorfor detecting the surface condition of a highway, runway or the like;

FIG. 2 is a schematic diagram of a surface condition detection systemutilizing the sensor of FIG. 1;

FIG. 3 is a perspective of a sensor utilized in a system of the presentinvention;

FIG. 4 is a schematic diagram of the present invention; and

FIG. 5 is a graph illustrating how the presence of salt in surfaceprecipitation affects the output of the sensor circuit in the system ofFIG. 4.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, asystem for detecting wet and icy conditions on the surface of highways,airport runways and the like is indicated in its entirety at l. A sensor2 for this system is shown in FIG. 1 as comprising a printed circuitboard 3 encapsulated in a block of synthetic resin material, such as anepoxy or the like. This is embedded in a highway or runway, etc., sothat the top of the block is substantially flush with the surfacethereof.

The insulative substrate of board 3 has a first pair of spaced-apart,interdigitated, substantially coplanar copper electrodes ClF, ClS etchedon its top surface and forming the electrodes ofa capacitor C1. On itsopposite surface the board has a pair of identical copper electrodesC2F, C2S (not shown in FIG. 1) in registry with respective electrodesClF and CIS and forming the plates of a second capacitor C2. The upperor top surface of board 3 is covered by a thin layer 4 of epoxy resin,e.g., l/l6 inch or so, so that electrodes ClF, ClS have only a thinlayer of epoxy covering them while the undersurface of the boardcarrying electrodes C2S, C2F has a much thicker layer (e.g. 1 inch)covering it.

This sensor unit is embedded in the surface of the roadway, highway,etc., and interconnected by a cable lead or by radio link to a remotemonitoring station. The use of interdigitated electrodes permits thesensor to be compact in size thus simplifying its installation in thesurface of a highway or the like.

With sensor 2 embedded in the surface so that the electrodes ofcapacitor C1 are substantially flush with the surface, capacitor C1 isexposed to atmospheric precipitation in that precipitation on thesurface forms part of its dielectric and accordingly affects itscapacitance. Capacitor C2 is not so exposed and precipitation does notaffect its dielectric or capacitance.

As shown in FIG. 2, identical but 180 out-of-phase signals which arecomposites ofa lower frequency substantially constant amplitude signaland a higher fre quency substantially constant amplitude signal areapplied to the first electrodes C1F,C2F of the capacitors. For thispurpose a signal generator 7 having, for example, a 200 Khz frequencyoutput and a low frequency (e.g., 1,000 Hz) signal generator 9 areseries connected from ground to electrode ClF. Conventional signalgenerators, preferably having the usual gain control to vary theamplitude of the output signals, may be used for this purpose.Typically, the signal generators are adjusted so that the amplitudes ofthe lower and the higher frequency signal components are equal. Thecomposite signal applied to electrode CIF is inverted by an inverter 11and a potentiometer P1 is provided to adjust the relative amplitudes ofthe 180 out-ofphase composite signals applied to capacitor electrodesClF and C2F. The second electrodes C15, C25 of the capacitors arecommonly connected.

Second electrodes C15, C25 are connected to the input terminal of aconventional operational amplifier Al, the output of which is filteredby a filter 13 to separate the 200 Khz and 1,000 Hz components of thecoupled, amplified composite signal. In the absence of any precipitationon the top of the exposed surface of sensor 2, the composite signalcoupled through the electrodes ClF, ClS of capacitor C1 to the input ofamplitier AI will be substantially equal to the amplitude of the 180out-of-phase composite signal coupled thereto via electrodes C2F, C2S ofcapacitor C2 inasmuch as the capacitors dielectric is constituted by thetwo layers 4 and 5 of the same epoxy resin material. As the thickness ofthe latter layer is considerably greater than that of the former,potentiometer P1 is adjusted to compensate for the effect of this minordifference in the capacitances of Cl, C2 under clear surface conditionsand insure that the resultant signal at the input of amplifier Al iszero. Thus, potentiometer P1 provides a means for balancing theamplitudes of the respective out-of-phase composite signals coupled tothe commonly connected second electrodes so that these signals willeffectively cancel each other when the exposed sensor surface is free ofprecipitation.

When the top surface of sensor 2 is covered with water, the dielectricconstant and thus the capacitance of capacitor C1 increases while thatof capacitor C2 remains unaffected. The resulting difference incapacitances causes a resultantcomposite signal to be present atcommonly connected second electrodes ClS, CZS. Since the dielectricconstant of water does not vary significantly with frequency (thegenerally accepted dielectric constant of water is about 78 at atemperature of 25C. at all frequencies from do to microwave), theamplitudes of the coupled higher and lower frequency components will beequal. The capacitance of C1 will also be affected by the formation ordeposit of frost, ice, sleet or snow on the exposed surface of sensor 2.While the dielectric constants of water and ice are the same at lowfrequencies, the dielectric constant of ice decreases as the frequencyis increased while that of water remains the same within limits. Ice, aswell as water, is made up of polar molecules and ice has a highdielectric constant at low frequencies. However, as the electricfrequency is increased, the onset of dielectric relaxation andconsequent decrease in dielectric constant occurs at a very much lowerfrequency in ice than in water. The frequency range over which thedielectric constant of ice shows a marked decrease is somewhat dependenton temperature; but for temperatures on the order of 0 to 10 F. atypical value is 2 to 50 Khz, i. e., the dielectric constant willdecrease as the frequency increases from 2 to 50 Khz. The polar natureof the water molecule is utilized herein. Thus, if the surface iscovered with ice, a resultant composite signal will be present at thesecond electrodes, but because of the phenomenon described above thelower frequency coupled component will be stronger than the higherfrequency coupled component because the dielectric constant of icedecreases markedly with frequency.

In summary then, if the surface is clear, no appreciable signal will bepresent at the commonly connected second electrodes. If the surface iswet, the 1.000 H2 and 200 Khz coupled components are approximatelyequal. But if the surface is icy, the amplitude of the 1,000 Hz coupledcomponent substantially will exceed that of the 200 Khz coupledcomponent.

Alternating current to direct current converters l5, l7 convert therespective 200 Khz and 1,000 Hz signals toanalogous d.c. voltages. Theoutput of converter 17 is commonly connected to one input of acomparator 19 and one input of a comparator 21. Comparator 19 has asecond input to which is applied a dc. voltage level adjustable by apotentiometer P2 connected from +V to ground. Comparator 19 serves toestablish a precipitation threshold. That is, if the output of converter17 is greater than an empirically determined threshold voltage, i.e., asadjusted by potentiometer P2, comparator 19 provides a logic 1 outputindicating some form of precipitation is present on the top surface ofsensor 2. On the other hand, if the output of converter 17 is below thed.c. level established by potentiometer P2, comparator 19 has a 0 outputindicating the surface is clear. it should be noted that 1,000 Hzcoupled component is used as an input to comparator 19 because it willhave a high amplitude regardless of whether water or ice is present onthe sensor top surface. Also inasmuch as any minor differences in thecapacitances of capacitors C1 and C2 under dry conditions may becompensated by adjustment of potentiometer P2, potentiometer P1 isoptional.

The output of converter is connected to the other input of comparator21. If both inputs to comparator 21 are of equal magnitude, it willprovide a logic 1 output indicating the surface is wet. If the output ofconverter 17 is greater than the output of converter 15, comparator 21has a logic 0 output indicating the surface is icy. Thus comparator 21is responsive to a change in the ratio of the magnitudes of the coupledhigher frequency signal component and lower frequency component toindicate icy conditions. Comparator 21 serves to determine whether thesurface is wet or icy once comparator 19 has determined some form ofprecipitation is on the surface.

A logic circuit 23 processes the outputs of the comparators and providessignals indicative of clear, wet, or icy surface conditions. The outputof comparator 19 is commonly connected to an inverter 25, to one inputof an AND gate G1, and to one input of an AND gate G2. The output ofcomparator 21 is commonly connected to the other input of gate G1 and tothe other input of gate G2 through an inverter 27.

Operation of the system of FIG. 2 is as follows: If the surface is dry,either no signal, or a very small signal, will be present at thecommonly connected second electrodes thereby causing the magnitude ofthe output of converter '17 to be less than the d.c. level applied tocomparator 19 by potentiometer P2. This in turn causes comparator 19 toprovide a0 logic signal which is inverted by inverter 25 which providesa 1 output in dicating clear surface conditions. If water is present onthe surface, a composite signal having substantially equal amplitudelower and higher frequency components will be applied to the input ofamplifier A1. The respective outputs of converters 15 and 17 will be ofsubstantially equal magnitude, that magnitude being greater than thed.c. level applied to comparator 19 by potentiometer P2. Each of thecomparators 19 and 21 will then have a l logic output thereby causinggate G1 to provide a 1 output which signals that the surface is wet.Finally, if the surface is ice covered, a resultant composite signalwith a lower frequency component of substantially greater amplitude thanthat of the higher frequency component will be coupled to sensor secondelectrodes CIS and C25 and the input of amplifier A1. The output ofconverter 17 will be of greater amplitude than either the output ofconverter 15 or the d.c. level applied to comparator 19 by potentiometerP2. This causes comparator 19 to provide a 1 output and com parator 21to have a 0 output which is inverted by inverter 27. With these inputsgate G2 will yield a 1 output thereby indicating an icy surfacecondition. The outputs of inverter 25 and gates G1 and G2 are employedto provide visual and/or audible indication of the various surfaceconditions. This system is able, then, to detect surface conditionswithout destroying the precipitation being detected through melting and-6 /or evaporation as is the case in systems using a conductivity sensorhaving heated electrodes.

Referring now to FIGS. 3 and 4, another surface condition detectionsystem is generally indicated at 29. A dual sensor 30 including printedcircuit board 31, similar to board 3 of the previous embodiment, isshown in FIG. 3 with a thin layer 32 covering the top of the p.c. boardand a thick layer 33 covering the bottom thereof. As in the previousembodiment, board 31 carries electrodes C1F,C1S of capacitor C1 on itstop surface and registering electrodes C2F, C2S of capacitor (2 on itsbottom surface. A pair of parallel spaced-apart strips of stainlesssteel 35, 37 which have their upper edges exposed through the top ofepoxy layer 32 are mounted to the p.c. board adjacent the electrodes ofcapacitor C1. Strips 35, 37 constitute the electrodes of an unheatedconductivity sensor 39. Sensor 30 is embedded in the surface aspreviously described. it should be noted that even with the parallelstrips added, sensor 30 is still of compact size and easily mounted inthe sur face of a highway, bridge deck, runway or the like.

The detection system of FIG. 4 includes a capacitance circuit 41, asensor circuit 43 and a logic circuit 45. Capacitance circuit 41functions to detect any atmospheric precipitation (either in the form ofwater or ice) present on the surface the condition of which is beingdetected. Sensor circuit 43 determines whether the precipitation is inthe form of water or ice. Logic circuit 45 is responsive to signals fromboth the capacitance and sensor circuits to indicate the state ofthesurface, viz., clear, wet, icy or alert. The significance of the alertindication will be explained hereinafter.

Capacitance circuit 41 is similar to the circuit described in theprevious embodiment except only the 200 Khz signal generator 7 is used.The junction between second electrodes ClS, C25 is connected to theinput of an alternating current to direct current con verter 47 theoutput of which is amplified by an ampli fier 49. Signal generator 7 andinverter 11 apply identical out-of-phase constant amplitude signals tofirst electrodes ClF, C2F respectively.

Potentiometer P1 is, as in the previous embodiment, optionally used toadjust the amplitude of the signal applied to electrode C2F so that inthe absence of any precipitation no substantial resultant signal iscoupled to the commonly connected second electrodes. The presence of anyprecipitation on the exposed surface of sensor 30 increases thecapacitance of C1 thereby causing a signal to be present at the secondelectrodes and a resulting d.c. voltage to be supplied at the output ofconverter 49. In this manner capacitance circuit 41 senses the presenceof any form of precipitation present on the surface' Sensor circuit 43determines, once capacitance circuit 41 has detected the presence ofprecipitation on the surface, whether that precipitation is in the formof ice or water. Sensor circuit 43 includes conductivity sensor 39 whichhas first and second spaced-apart electrodes 35, 37 exposed toprecipitation on the surface. Conductivity sensor 39 is seriesconnectedwith a resistor R3 across a battery 51. When a comparator 53, thepurpose of which will be explained below, has a 0 logic level output,the negative terminal of battery 51 is grounded thereby completing theseries circuit through sensor 39. The nature or state of theprecipitation on conductivity sensor 39 determines the potential orvoltage of the junction between sensor 39 and resistor R3 relative toground. This voltage, amplified by an amplifler 55, constitutes theoutput of sensor circuit 43. Since the conductivity of water is muchgreater than that of ice, sensor circuit 43 will provide a much loweramplitude output signal with water present on the surface than with icepresent.

Logic circuit 45 is responsive to the outputs of both capacitancecircuit 41 and sensor circuit 43 to provide indication of varioussurface conditions. The top input of a comparator 57 is connected to theoutput of amplifier 49 while its bottom input is connected to the sliderof a potentiometer P3 which applies an adjustable dc. voltage thereto.With any type of atmospheric precipitation present on the surface, themagnitude of the output d.c. signal level of amplifier 49 exceeds thatof the voltage applied by potentiometer P3 causing comparator 57 to havea 1 logic level output. On the other hand if the surface is free ofprecipitation, the output d.c. signal level of amplifier 49 will fail toreach the predetermined precipitation threshold magnitude as establishedby potentiometer P3 thereby causing comparator 57 to provide a output.

With uncontaminated water present on the surface of sensor 30, amplifier49 will have an exemplary output of 1 volt, while with uncontaminatedfrost the output of amplifier 49 will drop to a typical level of 0.4 to0.5 volts. With potentiometer P3 adjusted to apply a predeterminedprecipitation threshold voltage of a magnitude slightly less than 0.4volts to the bottom input of comparator 57, the comparator will providea 1 output if either water or ice is present on the surface of sensor30.

The output of amplifier 55 is commonly connected to respective topinputs of comparators 59, 61. Their bottom inputs are respectivelyconnected to the sliders of potentiometer P4, P5. Amplifier 55 will havean exemplary output of about 2.5 volts with ice present on the sensorssurface. With potentiometer P adjusted to supply slightly less than 2.4volts, a magnitude analogous to the resistance of the conductivitysensor with a mixture of ice and water on the surface, to comparator 61,this comparator will provide a 1 output when ice is present on thesurface. Potentiometer P4 is set so as to apply a slightly lower voltageto comparator 59 than potentiometer P5 applies to comparator 61. Whenwater on the surface begins to freeze, the output voltage level ofamplifier 55 exceeds the dc. voltage level supplied by potentiometer P4thereby causing comparator 59 to have a 1 output. The voltage levelsestablished by potentiometers P4, P5 at the inputs of comparators 59, 61respectively represent a predetermined alert threshold magnitude and apredetermined ice threshold magnitude.

The output of comparator 57 is commonly directly connected to aninverter 63 and to one input of each of three AND gates G4, G5 and G6.The output of comparator 59 is directly connected to one of the inputsof each of AND gates G5 and G6, and via an inverter 69 is connected toone of the inputs of gate G4. The output of comparator 61 is directlyconnected to the third input of gate G6 and, through inverters 71 and 73respectively, to the third inputs of gates G4 and G5.

Operation of the system of FIG. 4 is as follows: If the surface ofsensor 30 is dry either no signal, or a very small signal, will bepresent at the commonly connected second electrodes and the magnitude ofthe dc output of amplifier 49 is less than the dc. level applied 8 tocomparator 57 by potentiometer P3 thereby'causing comparator 57 to yielda 0 output which is inverted by inverter 63 to indicate a clear surfacecondition. If precipitation in any form is present on the sensor surfacea resultant net signal will be present on electrodes C 15. C28 causingthe d.c. output voltage of amplifier 49 to be greater than the dc. levelapplied to comparator 57 by potentiometer P3 and comparator 57 will havea 1 output. If that precipitation is in the form of water, theresistance of conductivity sensor 39 will be low causing the dc. outputvoltage of amplifier 55 to be lower than the dc. levels applied tocomparators 59, 61 by potentiometers P4, P5, respectively, which causescomparators 59, 61 to provide 0 outputs. With comparators 57, 59, 61having 1, 0, 0 outputs respectively, gate G4 will provide a 1 outputindicating the surface is wet. lf ice is on the surface, the resistanceof conductivity sensor 39 will be high causing the dc. output level ofamplifier 55 to be greater than the do. levels applied to comparators59, 61 by potentiometers P4, P5 respectively. This causes bothcomparators 59 and 61 to provide 1 outputs. With comparators 57, 59, 61each having a l output, gate G6 will in turn have a 1 output warningthat the surface is icy. Finally, if water on the surface is starting toturn into ice, the resistance of conductivity sensor 39 will be somewhatless than if only ice were present thereby causing the dc. output levelof amplifier 55 to be greater than the dc. level applied to comparator59 by potentiometer P4 but less than the dc. level applied to comparator61 by potentiometer P5. Thus comparator 59 will then provide a 1 outputand comparator 61 will have a 0 output. When comparators 57, 59, 61provide 1, 1,0 outputs respectively, gate G5 will have a 1 outputthereby effecting an alert signal indicating that ice is beginning toform. Thus the outputs of inverter 63 and gates G4, G5, G6 are employedto provide visual and/or audible indication of the various surfaceconditions.

The presence of ice control chemicals such as salt in precipitation onthe sensor surface affects the conduc tance of the precipitation. Lowconcentrations of salt, in the range of 0.25 up to 5 percent, willincrease the conductance of the thus contaminated precipitation coveringthe electrodes 35, 37 of conductivity sensor 39 considerably. Theconductance of salt water as compared to salt ice is quite different.Referring to P16. 5, as a comparison in the case of uncontaminated waterand uncontaminated ice, the voltage output of sensor circuit 43 variesfrom an exemplary level of about 1.5 volts for water to a 2.5 voltrepresentative level for ice (line AB), a delta increase of 1 volt. Inthe case of salt water-salt ice, a typical delta increase is in theorder of 0.75 to 0.8 volts (line DE) although the output levels ofsensor circuit 43 are considerably lower for salt water-salt ice thanfor water-ice.

The presence of ice control chemicals in the precipitation will alsoaffect the output level of capacitance circuit 41. As previouslyindicated with only frost or water present, the capacitance circuit hastypical output levels of 0.4 to 0.5 volts and 1 volt respectively.However, the output level of circuit 41 will increase to 1.2 volts wheresalt is present with ice and increase to 1.75 volts for salt water. Suchwas found to be true down to the lowest salt concentration tested (0.25percent).

The reducedoutput signal level (line DE of FIG. 5 of sensor circuit 43due to the presence of salt in the surface precipitation may becompensated with logic circuitry responsive to the marked increase ofthe output of capacitance circuit 41 when a salt is present inprecipitation on the surface. This is accomplished by increasing thevoltage applied across conductivity sensor 39 and resistor R3 so thatthe relationship of the output level of circuit 43 to surface conditionsis repre sented by line D'E' fo FIG. 5 which is substantially coincidentwith line AB.

To effect such a result, a salt compensating circuit includingcomparator 53 (see FIG. 4) is provided. One input of comparator 53 isconnected to the output of amplifier 49, the other input to the wiper ofa potentiometer P6 which is connected from to ground. The output ofcomparator 43 is connected to the negative terminal of battery 51.Potentiometer P6 is adjusted to apply slightly less than the exemplary1.2 volts to the top input of the comparator.

When the output signal level of capacitance circuit 41 exceeds thevoltage applied by potentiometer P6 to the other input of comparator 53,an output voltage is supplied which is additive to the voltage appliedby battery 51, across conductivity sensor 39 and resistor R3. Thisadditional voltage increment increases the magnitude of the output ofsensor circuit 43 thereby automatically to compensate for the increasedconductivity of conductivity sensor 39 caused by the presence of salt.Since potentiometer P6 applies a higher voltage to comparator 53 thanthe maximum output of the capacitance circuit under any no salt surfacecondition, comparator 53 can never function to supply any suchcompensating additional voltage increment when no salt is present in thesurface precipitation.

Again, the surface detection system of FIG. 4 does not change or affectthe nature or state ofthe precipitation being detected through meltingand/or evaporation. Moreover, this system of FIG. 4 operates reliablyeven under unusual surface conditions such as when a thick layer of icecovers the surface, when a very thin layer of moisture forms under anice layer as may occur with the use of ice control chemicals, or when alayer of water forms atop a layer of ice.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained,

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. A system for detecting wet and icy conditions on the surface ofhighways, airport runways and the like comprising:

a first capacitor adapted to be positioned on a surface the condition ofwhich is to be detected, said capacitor having first and secondspaced-apart electrodes adapted to be exposed to atmosphericprecipitation thereby to affect the dielectric and capacitance thereof;

a second capacitor comprising first and second spaced-apart electrodesadapted to be positioned so as not to be exposed to atmosphericprecipitation;

means for applying respective out-of-phase timevarying signals to thefirst electrodes of said capacitors, said second electrodes beingcommonly connected;

a sensor circuit comprising a conductivity sensor having first andsecond spaced apart electrodes adapted to be exposed to atmosphericprecipitation thereby to affect the resistance thereof, the resis tanceof said conductivity sensor being greater in the presence of ice than ofwater, said sensor circuit adapted to supply an output voltage themagnitude of which is a function of the resistance of said conductivitysensor; and

an electrical logic circuit responsive to any signal coupled to thecommonly connected second electrodes of the capacitors reaching apredetermined precipitation threshold magnitude and to the outputvoltage of said sensor circuit reaching a predetermined ice thresholdmagnitude to provide an output which indicates an icy surface condition.

2. A system for detecting wet and icy conditions as set forth in claim 1wherein said logic circuit includes means responsive to the magnitude ofthe coupled signal reaching said precipitation threshold magnitude andthe sensor circuit output voltage failing to reach said ice thresholdmagnitude to provide a second output which indicates a wet surfacecondition.

3. A system for detecting wet and icy conditions as set forth in claim 2wherein said logic circuit further includes means responsive to themagnitude of coupled signal failing to reach said precipitationthreshold magnitude to provide a third output which indicates a clearsurface condition.

4. A system for detecting wet and icy conditions as set forth in claim 3wherein said logic circuit includes means responsive to the magnitude ofthe coupled signal reaching said precipitation threshold magnitude andthe sensor circuit output voltage reaching a predetermined alertthreshold magnitude but failing to reach said ice threshold magnitude,the alert threshold magnitude of voltage being analogous to theresistance of said conductivity sensor with a mixture of ice and wateron said surface, to provide a fourth output which indicates theinception of the formation of ice on said surface.

5. A system for detecting wet and icy conditions as set forth in claim Ifurther comprising means for compensating the output voltage of saidsensor circuit for the presence of a salt in any precipitation on saidsurface, the presence of a salt decreasing the resistance of saidconductivity sensor, whereby the magnitude of the output voltage of saidsensor circuit is substantially unaffected by the presence of salt inany precipitation on said surface, said compensating means beingresponsive to the magnitude of the signal coupled to said commonlyconnected second electrodes of the capacitors, the magnitude of thecoupled signal increasing substantially with the presence of salt inprecipitation on said surface.

a. A system for detecting wet and icy conditions as set forth in claim 1wherein said respective signals are substantially indentical andout-of-phase.

'7. A system for detecting wet and icy conditions as set forth in claim6 further comprising means for balancing the signals coupled to saidsecond electrodes of each of said capacitors so that in the absence ofany precipitation substantially no resultant signal is present at thesecond electrodes.

8. A system for detecting wet and icy conditions as set forth in claim 7wherein the balancing means includes means for adjusting the magnitudeof the signal applied to the first electrode of one of the capacitors. l=l l= =l

1. A system for detecting wet and icy conditions on the surface of highways, airport runways and the like comprising: a first capacitor adapted to be positioned on a surface the condition of which is to be detected, said capacitor having first and second spaced-apart electrodes adapted to be exposed to atmospheric precipitation thereby to affect the dielectric and capacitance thereof; a second capacitor comprising first and second spaced-apart electrodes adapted to be positioned so as not to be exposed to atmospheric precipitation; means for applying respective out-of-phase time-varying signals to the first electrodes of said capacitors, said second electrodes being commonly connected; a sensor circuit comprising a conductivity sensor having first and second spaced apart electrodes adapted to be exposed to atmospheric precipitation thereby to affect the resistance thereof, the resistance of said conductivity sensor being greater in the presence of ice than of water, said sensor circuit adapted to supply an output voltage the magnitude of which is a function of the resistance of said conductivity sensor; and an electrical logic circuit responsive to any signal coupled to the commonly connected second electrodes of the capacitors reaching a predetermined precipitation threshold magnitude and to the output voltage of said sensor circuit reaching a predetermined ice threshold magnitude to provide an output which indicates an icy surface condition.
 2. A system for detecting wet and icy conditions as set forth in claim 1 wherein said logic circuit includes means responsive to the magnitude of the coupled signal reaching said precipitation threshold magnitude and the sensor circuit output voltage failing to reach said ice threshold magnitude to provide a second output which indicates a wet surface condition.
 3. A system for detecting wet and icy conditions as set forth in claim 2 wherein said logic circuit further includes means responsive to the magnitude of coupled signal failing to reach said precipitation threshold magnitude to provide a third output which indicates a clear surface condition.
 4. A system for detecting wet and icy conditions as set forth in claim 3 wherein said logic circuit includes means responsive to the magnitude of the coupled signal reaching said precipitation threshold magnitude and the sensor circuit output voltage reaching a predetermined alert threshold magnitude but failing to reach said ice threshold magnitude, the alert threshold magnitude of voltage being analogous to the resistance of said conductivity sensor with a mixture of ice and water on said surface, to provide a fourth output which indicates the inception of the formation of ice on said surface.
 5. A system for detecting wet and icy conditions as set forth in claim 1 further comprising means for compensating the output voltage of said sensor circuIt for the presence of a salt in any precipitation on said surface, the presence of a salt decreasing the resistance of said conductivity sensor, whereby the magnitude of the output voltage of said sensor circuit is substantially unaffected by the presence of salt in any precipitation on said surface, said compensating means being responsive to the magnitude of the signal coupled to said commonly connected second electrodes of the capacitors, the magnitude of the coupled signal increasing substantially with the presence of salt in precipitation on said surface.
 6. A system for detecting wet and icy conditions as set forth in claim 1 wherein said respective signals are substantially indentical and 180* out-of-phase.
 7. A system for detecting wet and icy conditions as set forth in claim 6 further comprising means for balancing the signals coupled to said second electrodes of each of said capacitors so that in the absence of any precipitation substantially no resultant signal is present at the second electrodes.
 8. A system for detecting wet and icy conditions as set forth in claim 7 wherein the balancing means includes means for adjusting the magnitude of the signal applied to the first electrode of one of the capacitors. 